Fix label names when internalSignature has optional params

[aswinsvijay]

Fixes #1614

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Unsure how this can be tested since tuple labels are not part of the type system.

[pullfrog]

Reviewed PR #1615 — approved. The one-line fix correctly wraps Parameters<internalSignature> with Required<> to prevent optional implementation parameters from breaking label inference in applyElementLabels.

Task list (5/5 completed)

• Read the PR diff and understand the change
• Look at surrounding context in ark/type/fn.ts
• Check the linked issue fn does not infer parameter labels correctly if internalSignature has optional parameters #1614 for context
• Investigate applyElementLabels to understand impact of Required<>
• Submit review

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[pullfrog]

Reviewed — no issues found.

The fix is correct: Required<Parameters<internalSignature>> ensures the labels tuple passed to applyElementLabels is always fully required, which is the right semantics since parameter optionality in the external signature is determined by distill.In<paramsT> (the first arg), not the implementation's parameter declarations. The recursive [unknown, ...infer labelsTail] destructuring in applyElementLabels can break when the labels tuple has optional elements — this cleanly prevents that.

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[pullfrog]

Reviewed — no issues found.

The fix is correct: Required<Parameters<internalSignature>> ensures the labels tuple passed to applyElementLabels is always fully required, which is the right semantics since parameter optionality in the external signature is determined by distill.In<paramsT> (the first arg), not the implementation's parameter declarations. The recursive [unknown, ...infer labelsTail] destructuring in applyElementLabels can break when the labels tuple has optional elements — this cleanly prevents that.

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